Seismogenic Structure of the April 20, 2013, Lushan Ms7 Earthquake in Sichuan

At 08:02 on April 20, 2013, a Ms7.0 earthquake occurred in Lushan, Ya'an, in the Longmenshan fault zone, Sichuan. The epicenter was located between Taiping Town and Shuangshi Town, Lushan County and the maximum earthquake intensity at the epicenter reached class IX. Field investigations in the epicenter area found that, although buildings were seriously damaged, no obvious surface rupture structure was produced, only some ground fissures and sand blows and water ejection phenomena being seen. An integrated analysis of high-resolution remote sensing image interpretation, mainshock and aftershock distribution, and focal mechanism solutions indicated that this earthquake was an independent rupturing event in the southwestern segment of the Longmenshan fault zone, belonging to the thrust-type earthquake. Ruptures occurred along the south-central segment of the Shuangshi-Dachuan fault and the principal rupture plane dipped SW at 33-43°. It is inferred that the Lushan earthquake might be related to the ramp activity of the basal detachment zone (13-19 km) of the Longmenshan fault zone. Historically, there occurred at least two Ms6-6.5 earthquakes along the Shuangshi-Dachuan fault zone; thus it is thought that the Lushan earthquake, different from the Wenchuan earthquake, was a characteristic one in the southwestern segment of the Longmenshan fault zone. In-situ stress measurements indicated the Lushan earthquake was the result of stress release of the southwestern segment of the Longmenshan fault zone after the Wenchuan earthquake. This paper analyzes the tectonic setting of the seismogenic structure of this earthquake.     

芦山地震发震构造及其与汶川地震关系讨论

芦山地震发生在巴彦喀拉块体与华南块体之间龙门山推覆构造带南段。野外考察表明,芦山地震在震中区没有形成具有构造地质意义的地震地表破裂带,仅在各山前陡坡地带出现平行于山麓陡坡的张性地裂缝、山地基岩崩塌、滑坡等边坡震动失稳现象和震动引起的砂土液化现象。重新定位的芦山地震余震分布、震源机制解和地表构造地质等分析表明,芦山地震的发震断层为一条现今尚未出露地表、其上断点仍埋藏在地下9 km以下地壳中的一条盲逆断层,走向212°,倾向NW,倾角38°±2°,上断点以上至地表的构造变形符合断层扩展背斜模型。根据汶川地震和芦山地震的余震空间分布、地震破裂过程、深浅构造关系等差异反映出它们是分别发生在龙门山推覆构造带中段和南段的两次独立地震破裂事件。     

汶川、芦山地震前龙门山断裂带地壳形变特征对比分析

为研究2008年汶川地震和2013年芦山地震前地壳形变特征,本文利用1999—2015年四期GPS速度场和1990—2017年跨断层短水准资料,对跨断层GPS速度剖面、GPS应变率场、断层闭锁程度和滑动亏损、跨断层年均垂直变化速率等进行了分析讨论,总结了汶川和芦山地震前后龙门山断裂带三维地壳变形演化特征。结果表明,汶川地震前龙门山断裂带中、北段处于强闭锁状态、断层面应力应变积累水平很高,而龙门山断裂带西南段闭锁较弱、变形速率明显高于中北段、依然可以积累应力应变,汶川地震震源位于闭锁相对弱的部位,这可能是导致汶川地震自初始破裂点沿龙门山断裂带向北东方向单侧破裂,而震中西南方向断层并没有发生破裂的原因之一。汶川地震的发生引起龙门山断裂带西南段应力应变积累速率加快、断层闭锁程度增强、闭锁面积增大,这在一定程度上促进了芦山地震的发生,而芦山地震震源位于汶川地震前强闭锁和弱闭锁的高梯度过渡部位。因为芦山地震只释放了龙门山断裂带西南段有限的应变能,并没有显著缓解该段的地震危险性,所以汶川和芦山地震之间的地震空段以及芦山地震西南方向的地震空段,依然需要持续关注。此外,本文还收集和对比分析了多次6～9级地震前地壳变形特征,同样显示地震成核于闭锁高梯度带区域而非完全闭锁区域内部,并且随着震级升高闭锁断层面的长度也在增大,这一现象还需在高分辨率形变数据的帮助下进行深入研究和分析。     

The 2014 Northern Thailand Mw 6.1 Earthquake and its Seismogenic Tectonics

On May 5, 2014, an earthquake with a magnitude of Mw 6.1 (the largest earthquake in Thailand so far) occurred in Chiang Rai of the Golden Triangle area in northern Thailand. We had an opportunity to conduct field survey immediately after the earthquake. Serious damage to buildings and casualties of lives were observed, and the estimated Maximum Mercalli Intensity (MMI) of the earthquake is VIII (evaluated according to the MMI scale of the Chinese Standard). No long continuous ground ruptures were produced during the earthquake,??but in the epicenter (commonly within MMI VIII extent), massive small linear ruptures (usually several tens of meters long) developed and displayed intriguing structural features, offsetting many roads several centimeters left laterally on NE trending cracks or offsetting right laterally on NW trending ones. The focal mechanism solution of earthquake shows that this is a pure strike-slip event, and two nodal planes in NW and NE directions had the same motion senses respectively as those of breakage associated with the earthquake. The long axis of the isoseismals and aftershock distributions are in NE direction,which is consistent with the strike of Luang Namtha fault. The 230-km-long Luang Namtha fault which starts from the border of China and Laos, runs through northern Laos, and terminates at Chiang Rai of Thailand is predominated by left-lateral strike-slip and active in late Quaternary, and two earthquakes over Ms 6.0 occurred along the fault in 1925 and 2007 respectively. This Mw 6.1 earthquake occurred at the southwestern end of the fault. All related features such as evident structural rupturing, elongated orientation of MMI and aftershock distribution,as well as the location of the epicenter,suggest that the Luang Namtha fault may be responsible for the 2014 Northern Thailand earthquake.     

四川芦山2013年Ms7.0地震发震构造初步研究

2013年4月20日8时2分,四川龙门山断裂带的雅安芦山发生Ms7.0级地震,震中位于芦山县太平镇和双石镇之间,震源深度13～14km,震中最大烈度达IX级。震中区野外调查发现,尽管房屋建筑损坏较严重,但这次地震没有产生明显的地表破裂构造,仅见少量的地裂缝和喷砂冒水现象。高分辨率遥感图像解译、主余震分布、震源机制解等综合分析认为,该地震是龙门山断裂带西南段一次独立的破裂事件,属于逆冲型地震,沿双石-大川断裂中南段发生破裂,主破裂面西倾,倾角33°～43°,推断芦山地震与龙门山构造带底部滑脱带(13～19km)断坡构造活动有关。历史上,沿双石-大川断裂发生至少2次Ms6～6.5级地震,由此认为芦山地震是龙门山断裂带西南段特征型地震,与汶川地震不同。原地地应力测量和监测数据表明这是汶川地震后龙门山断裂带西南段应力释放的结果。     

Deep Seismogenic Environment in the Southern Section of the Longmenshan Fault Zone on the Eastern Margin of the Tibetan Plateau and Lushan Ms 7.0 Earthquake

The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20 th,2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional(3D) P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric(MT) sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover,at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya’an,the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Lowvelocity anomalies are distributed underneath the are of seismogenic origin,The inversion results of the MT retest data after the April 20 th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake,Therefore,we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and played a triggering role in the Lushan earthquake.     

Seismogenic Structure around the Epicenter of the May 12, 2008 Wenchuan Earthquake from Micro‐seismic Tomography

Abstract: A three-dimensional local-scale P-velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of ~2 km. The high-resolution P-velocity model revealed interesting structures in the seismogenic layer: (1) The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; (2) The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least ~10 km from the surface; (3) A sharp lateral velocity variation beneath the Wenchuan-Maoxian fault may indicate that the Pengguan massif’s western boundary and/or the Wenchuan-Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu-Beichuan and Guanxian-Anxian faults, and vertical Wenchuan-Maoxian fault; (4) Vicinity along the Yingxiu-Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; (5) Two blocks of low-velocity anomaly are respectively imaged in the hanging wall and foot wall of the Guanxian-Anxian fault with a ~7 km offset with ~5 km vertical component.     

基于GPS监测的芦山地震周围地区同震位移与运动速度

通过对2013年"4.20"四川芦山地震前后GPS观测数据的处理,得到地震周围地区GPS测站同震位移及速度矢量场。GPS测站同震位移大小为5.09~51.05mm,平均为14.18mm;GPS测站运动速度为2.64~52.37mm/a,平均为18.89mm/a。利用断裂两侧GPS测站速度矢量差得到了龙门山断裂带南段次级断裂的运动速度,龙门山断裂带南段的后山断裂、中央断裂、前山断裂运动速度大小分别为49.66 ±3.90mm/a、79.58 ±3.33mm/a、50.94 ±3.91/a;中央断裂以右旋挤压为主,而后山断裂、前山断裂表现为左旋拉张的特性。综合分析表明,芦山地震是发生在龙门山断裂带南段东南侧的逆冲型地震,发震构造为前山断裂与新津断裂之间的小断层。芦山地震对周围地区的影响不大,主要集中在龙门山断裂带南段及震中附近区域。     

西藏米林6.9级地震发震断层判定及其构造属性讨论

西藏米林6.9级地震是1950年察隅MS8.6级地震以来东构造结地区发生的第一个6级以上地震,由于地震发生在雅鲁藏布江大拐弯附近的无人区,野外应急科学考察没有发现明显的地震地表破裂带。根据林芝流动地震观测台网记录到的余震条带状分布、震后科学考察获得的地震烈度长轴方位、原有台阵观测震中分布以及GPS资料和野外地质调查分析,发现发震断层为NW向排龙—帮辛断裂,逆冲性质,为新生NW向边坝—达木断裂带中段组成部分;米林地震的发生预示东构造结地区地震活动强度加强,未来该地区发生7级以上地震危险性不容忽视。     

青藏高原东缘1933年叠溪Ms7.5级地震发震构造再研究

青藏高原东缘1933年叠溪75级地震的发震构造至今仍然难以琢磨,前人或将其归因于NW向松坪沟断裂的左旋走滑活动、或南北向岷江断裂左旋走滑活动,但地表同震破裂、地震地质、地震等烈度图等调查和研究结果都不支持这种走滑型断层的地震成因。本文基于叠溪地震区构造地貌和湖相地层断层调查,结合古地震和历史地震的研究结果,提出了与2013年四川芦山Ms 70级地震类似的发震构造模型,即隐伏断坡型逆冲断层发震构造模型,认为在叠溪震区10~15km深部隐伏一条西倾的逆冲断坡,其向东逆冲作用导致了叠溪地区频繁的地震活动。这个发震模型有待深部地球物理测深资料和地表大地测量资料的验证。     

芦山7.0级地震与巴颜喀拉块体中东段的活动性

巴颜喀拉块体及周缘是近年来中国大陆大震主要发震区域之一,2013年4月20日四川省芦山7.0级强烈地震就发生在巴颜喀拉块体东缘的龙门山断裂带的西南段上。以巴颜喀拉块体中东段为研究区,通过对该区域地震前后的多期区域水准观测数据和GNSS连续站观测数据进行计算处理,得出研究区垂直形变速率图像和水平形变速率图像;结合区域地质构造,建立地震诱发模型,解释芦山地震主震发震和余震活动源于龙门山断裂带中大型块体的突然断裂和小型块体集合的流动机制;通过对垂直形变速率图像和水平形变速率图像的分析,认为芦山地震是龙门山断裂带逆冲活动的结果,东昆仑断裂带中东段未来大震发生的可能性增大;提出未来5年基于区域水准测量和GNSS测量为技术手段的监测建议。     

四川汶川Ms 8.0级地震的地表变形与同震位移

四川汶川Ms 8.0级强烈地震与青藏高原东部松潘-甘孜地块东向挤出导致的龙门山活动断裂右旋斜冲运动存在动力学成因联系。沿龙门山中央北川-映秀断裂发育长度超过250km的地震变形带,由地震陡坎、地震鼓包、地震破裂、地震断层组成,形成了较大的同震位移。在震中区映秀观测到的最大同震位移为7.6m,由右旋走滑位移6.1m和垂直位移4.6m 2个分量组成;虹口地区的右旋走滑位移为2.7m,垂直位移为4.6m,右旋斜冲总位移为5.3m;北川地区的右旋走滑位移为5.7m,垂直位移为3.4m,右旋斜冲总位移为6.6m;平通地区的右旋走滑位移为3.2m,垂直位移为3.0m,右旋斜冲总位移为4.4m。龙门山前缘的汉旺-漩口断裂及龙门山后缘的茂县-汶川断裂、青川断裂也发生了显著的同震断裂活动,但同震位移小于等于1.0m。根据同震位移实测数据和构造会聚速率的GPS观测资料,估算龙门山地区8.0级地震的复发周期为1150~2950年。     

芦山地震前后龙门山断裂带西南段地应力状态对比分析

2013年4月20日芦山Ms7.0级地震后,为研究龙门山断裂带西南段震后的地应力状态,应用水压致裂法和压磁应力解除法在该区开展了2个钻孔的原地应力测量工作。测量结果显示硗碛测点在128～188m深度范围内最小水平主应力的量值为10.47～18.47 MPa,最大水平主应力的量值为19.60～25.83 MPa,方向为N63°～85°W;天全测点在114～142m深度范围内最小水平主应力的量值为5.20～7.73 MPa,最大水平主应力的量值为8.21～9.31 MPa,方向为N59°W。两个测点水平主应力与垂直应力的关系均为σ_Hσ_hσ_v,其中硗碛测点最大、最小水平主应力与垂直应力比值的平均值分别为5.27和3.01,天全测点最大、最小水平主应力与垂直应力比值的平均值分别为2.60和1.76,表明有利于逆断层活动。通过比较该地区芦山地震前后实测地应力状态,发现芦山地震后,龙门山断裂带西南段的北段(即邛崃大邑西-宝兴北-汶川南一带)和南段(即天全-荥经-泸定-康定一带)应力积累量增加。相同深度范围内,北段硗碛测点震后的应力大小要比地震前有明显的提高,这也与硗碛测点地应力监测结果一致。实测应力方向与震前基本一致,都为NW-NWW。基于实测地应力资料,根据库伦破裂准则和Byerlee定律分析,位于北段的硗碛测点震前部分压裂段的最大水平主应力处于使断层滑动临界值的上下限之间,而地震后最大水平主应力则均已超过断层滑动临界值的上限。位于南段的飞仙关测点震前最大水平主应力均未达到断层滑动临界值的下限,而地震后天全测点的最大水平主应力则均处于使断层滑动临界值的上下限之间。采用最大剪应力(σ_1-σ_3)/2与平均应力(σ_1+σ_3)/2的比值μ_m(断层摩擦)参数评估研究区地应力的积累水平和地震危险性。震前硗碛测点μ_m的量值为0.16～0.72,平均为0.50,震后为0.71～0.81,平均为0.77。震前飞仙关测点μ_m的量值为0.31～0.35,平均值为0.32,震后天全测点μ_m的量值为0.53～0.57,平均值为0.55,两个研究区的μ_m的量值均变大。分析认为芦山地震后龙门山断裂带西南段的北段和南段的应力积累量增加,都有发生断层滑动的可能性,尤其是北段。     

Seismogenic Structure around the Epicenter of the May 12,2008 Wenchuan Earthquake from Micro-seismic Tomography

A three-dimensional local-scale P-velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network.Checkerboard tests show that our tomographic model has lateral and vertical resolution of～2 km.The high-resolution P-velocity model revealed interesting structures in the seismogenic layer:(1) The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian f...     

2014年于田Ms73地震地表破裂特征及其发震构造

2014年2月12日在新疆于田县境内西昆仑山东段地区发生了Ms7.3级强烈地震,震后野外考察表明,这次地震在海拔4600~5100m的地区形成了由一系列张裂隙、张剪裂隙、剪切裂隙以及挤压鼓包和裂陷等雁行状组合而成的地表破裂带,破裂带沿阿尔金断裂带西南段的两条近平行的分支断裂阿什库勒-硝尔库勒断裂和南硝尔库勒断裂分布,整体呈NEE走向,全长约28km,其中,沿阿什库勒-硝尔库勒断裂展布的地表破裂带长约10km,主要呈N63°~65°E走向,以左旋走滑伴随伸展性质的破裂为主,最大左旋位移约0.7m,最大垂直位移约0.4m;沿南硝尔库勒断裂展布的地表破裂带长约15km,呈N54°~60°E走向,以左旋走滑伴随逆冲性质的破裂为主,最大左旋位移约1m,最大垂直位移约0.75m;上述两破裂带之间沿N15°E方向由零星的张裂隙和右阶雁行状分布的张裂隙或张剪裂隙组成的不连续破裂带长约5km,显示为伸展具有左旋走滑的性质;另外,在南硝尔库勒断裂北侧沿N100°~110°E方向展布一系列具有挤压、右旋走滑性质的地表破裂带长约4km,宽约2km,与NEE走向的左旋走滑破裂带构成同震共轭破裂带。这种特殊的地表破裂样式是近期发生的强地震中结构最复杂的走滑断层型地表破裂。发震断裂属于阿尔金断裂带西南段尾端分支断裂,它与郭扎错断裂和龙木错断裂构成"阿尔金断裂"向SW方向的延伸部分,它们是青藏高原西部晚新生代强烈活动断裂,其大地震活动是由于印度和欧亚板块间碰撞而产生大陆变形的应变能释放过程。     

2013年四川芦山地震次生山地灾害发育规律

2013年4月20日8时02分,青藏高原东缘龙门山南部地区发生了芦山地震(Ms 7.0),此次地震诱发了大量次生山地灾害。以地震重灾区宝兴作为研究区,利用卫星遥感影像、数字高程模型和高清航拍图像,以及崩塌滑坡数据统计分析,并结合野外调查研究,对区内次生山地灾害的空间分布与岩性、断裂和坡度关系进行了分析和探讨,总结了宝兴地区地震诱发的次生山地灾害发育规律:1以中小型崩塌滑坡为主,且沿省道S210集中分布;2崩塌滑坡主要发生在宝兴杂岩区浅表强风化层及第四系松散堆积层;3研究区内发育的五龙断裂和小关子断裂不是芦山地震的同震断裂;4该区域70%的崩塌滑坡发生在坡度大于30°的区域范围内,30~40°坡度段崩塌滑坡最为集中;5人类工程活动是宝兴地区次生山地灾害集中发育在S210省道两侧的主要原因;6在汶川地震和芦山地震2次地震及其余震的频繁加载作用下,宝兴地区崩塌滑坡的活动性增加,未来几年将是中小规模崩塌滑坡发育的高峰期。     

2014年5月云南盈江M_S5.6、M_S6.1地震发震构造分析

2014年5月云南省盈江县先后发生MS5.6、MS6.1地震,为确定它们的发震构造及其所反映的区域活动构造格局,笔者围绕该区开展了地震烈度调查、活动构造遥感解译、地质构造及构造地貌野外调查、震源机制解及余震分布资料分析等工作。调查与分析表明,两次地震的宏观震中均位于盈江县勐弄乡麻栗坡村附近,但发震断层明显不同。前者为NE走向左旋走滑的昔马—盘龙山断裂,后者为近SN向右旋走滑的苏典断裂。历史地震资料显示,盈江地区的地震活动多以5~6级的中-强震为主,并具有明显的群发性和沿SN向断层迁移的特征。在实皆断裂及滇西内弧带的共同作用下,腾冲地块内以大盈江断裂为界,北部主要发育近SN向右旋走滑断裂,南部则以NE向左旋走滑断裂为主,其中近SN向断层晚第四纪活动性更强。